I think daffy is on the right track. When the print queue is stopped in OS X, it takes a manual press of the start button to get going again. It can get stopped if the button is accidently pushed, the printer runs out of paper or ink in the middle of printing, or you cancel your print job in the middle. Kinda sensitive if you ask me.
Every time it happens, I just open the Print Center and start up the queue. Works every time.